Global Conformational Rearrangements in Integrin Extracellular Domains in Outside-In and Inside-Out Signaling

How ligand binding alters integrin conformation in outside-in signaling, and how inside-out signals alter integrin affinity for ligand, have been mysterious. We address this with electron microscopy, physicochemical measurements, mutational introduction of disulfides, and ligand binding to αVβ3 and αIIbβ3 integrins. We show that a highly bent integrin conformation is physiological and has low affinity for biological ligands. Addition of a high affinity ligand mimetic peptide or Mn2+ results in a switchblade-like opening to an extended structure. An outward swing of the hybrid domain at its junction with the I-like domain shows conformational change within the headpiece that is linked to ligand binding. Breakage of a C-terminal clasp between the α and β subunits enhances Mn2+-induced unbending and ligand binding.